The invention relates to a method for producing a decarburised hot-rolled strip which is made of a heat-treatable steel containing at least 0.4% by weight of carbon. Hot-rolled strip produced from steels of this kind is of high hardness and is therefore particularly suitable for the manufacture of articles which are subject to high but locally limited loads in use. This is the case with for example punching blades and comparable cutting tools where, in practical use, both the cutting edge and also the body of the blade which carries the cutting edge have to withstand high forces during the cutting process.
The advantages of using heat-treatable steels having comparatively high carbon contents are offset by the disadvantage that, due to their high hardness, it is only with comparative difficulty that steels of this kind can be subjected to forming processes. This means that when for example thin sheets produced from heat-treatable steel of high hardness are subjected to a forming process the formation of cracks occurs at the surface of the sheets and these cracks may then be the starting point of fractures in the component produced from a given sheet.
Basically, it is known that the formability of steels can be improved by decarburising annealing. In this way, steels intended for deep drawing which are produced from soft steels are subjected to decarburising annealing. The aim in this case is to reduce the carbon content as uniformly as possible across the entire cross-section of the sheet or plate in order to ensure that its behaviour when formed is as uniform as possible.
An example of the decarburising annealing of a cold-rolled strip intended for processing by deep drawing which is made of a steel having flow carbon contents which are typically appreciably less than 0.03% by weight is described in GB 1,189,464. In this known process, hot-rolled strip is hot rolled from a slab at a final rolling temperature of 850-950° C. The hot-rolled strip obtained is then reeled at a reeling temperature of 600° C. and is then cold rolled to the desired final thickness.
After the cold rolling, the cold-rolled strip is wound into an open coil by the known method and is decarburising annealed as an open coil. An open coil of this kind is wound sufficiently loosely for the individual layers of the coil which it forms to be separated from one another by spaces. In this way, the reactive gas can flow through the gaps present between the individual layers of the coil, which means that in case the flow of gas is guided in an optimised way, the gas is allowed to sweep over every surface of the coil in the same way.
To achieve the degree of decarburisation which is required in GB 1,189,464, long annealing times are required. In this way, a steel containing 0.04% by weight of carbon has, under GB 1,189,464, first to be heated for 8 to 12 hours to the decarburising annealing temperature required in a substantially dry atmosphere. Water vapour is then fed into the atmosphere of the furnace in a ratio of 200:1 to set the decarburising process in motion. The decarburising annealing is then continued for a further 10 hours in the reducing atmosphere which has been formed in this way until the desired reduction in the carbon content has been achieved.
Another possible way of decarburising annealing cold-rolled strip intended for deep-drawing purposes which contains 0.03-0.06% by weight of carbon is described in DE-OS 2 105 218. In this method, the cold-rolled strip is passed in a continuous run through a furnace in which a reducing atmosphere is maintained at an annealing temperature which is less than 780° C. The time of passage of the strip through the annealing furnace is set in such a way that when it emerges from the annealing furnace its carbon content is less than 0.01% by weight.